The use of methylenephosphonic acid substituted alkylene polyamines for metal ion control at less than stoichiometric amounts was suggested in a patent to Bersworth (U.S. Pat. No. 2,609,390) in 1952. Later a water dispersible polymeric amine chelating agent which included alkylene phosphonate derivatives was indicated as having "threshold" effects in scale inhibition applications (U.S. Pat. No. 3,331,773), this term being used to describe the use of the agent in less than stoichiometric amounts. The diamine and polyamine methylenephosphonate derivatives are taught and claimed in U.S. Pat. Nos. 3,336,221 and 3,434,969, respectively. Some of the products disclosed in these two patents are available commercially and are recommended as scale inhibitors when applied in threshold amounts.
Some other patents which disclose heterocyclic nitrogen containing compounds which are useful as chelating agents and may be employed in threshold amounts are U.S. Pat. Nos. 3,674,804; 3,720,498; 3,743,603; 3,859,211; and 3,954,761. Some of the compounds included therein are heterocyclic compounds having the formulas: ##STR5## wherein R is hydrogen or alkyl and M is hydrogen, alkali metal, ammonium or a di- or triethanolamine radical; ##STR6##
Methylenephosphonates of polyalkylene polyamines, disclosed in U.S. Pat. No. 4,051,110, are made by reacting di- or polyamines with a chain extending agent such as a dihalide or an epoxyhalide, e.g. ethylene dichloride or epichlorohydrin and thereafter, with phosphorus acid and formaldehyde. Thus, for example, triethylenetetramine is reacted with epichlorohydrin in an approximately one to one mole ratio; thereafter the product is reacted with phosphorous acid, and formaldehyde in the presence of hydrochloric acid. The resulting methylenephosphonated polyamine is useful in small amounts as a scale inhibitor, being employed at concentrations of 20-50 ppm.
Certain phosphonic acid derivatives of the aliphatic acids can be prepared by reacting phosphorous acid with acid anhydrides or acid chlorides, e.g. the anhydrides or chlorides of acetic, propionic and valeric acids. The compounds prepared have the formula ##STR7## wherein R is a lower alkyl radical having 1 to 5 carbon atoms. The method of making and use of these products is described in U.S. Pat. No. 3,214,454. It discloses and claims the use of threshold amounts to prevent calcium precipitation in aqueous solutions.